Shadow masks for use in colour picture tubes

ABSTRACT

In a shadow mask for use in a colour picture tube of the type including a plurality of perforations for passing an electron beam there are provided on the opposite sides of the shadow mask layers of a substance capable of suppressing the emission of secondary electrons caused by the collision of the electron beam.

United States Patent Yamazaki et al.

1 1 Sept. 18, 1973 SHADOW MASKS FOR USE IN COLOUR PICTURE TUBES Eiichi Yamazaki, lchihara; l-liromi Kanai, Mobara, both of Japan Hitachi Ltd., Tokyo, Japan Jan. 3, 1972 Inventors:

Assignee:

Filed:

Appl. No.:

U.S. Cl 313/85 S, 313/92 B, 313/107 Int. Cl. HOIj 29/46 Field of Search 313/85 S, 92 B, 107

References Cited UNITED STATES PATENTS 12/1955 Burnside 313/92 B X 10/1959 Nordberg 313/85 S 2,942,130 6/1960 Sheldon 313/85 8 2,971,117 2/1961 Law 3,136,916 6/1964 Schaefer.....

3,398,309 13/1968 Kaplan 3,555,334 1/1971 De Goor 313/107 Primary Examiner-John Kominski Assistant Examiner-James B. Mullins Attorney-Chittick, Thompson & Pfund 57- ABSTRACT- 4 Claims, 4 Drawing Figures PATENTED SEP] 8 I975 BACKGROUND OF THE INVENTION This invention relates to an improved shadow mask for use in a colour picture tube, more particularly a colour receiving tube of the post focusing type.

For the purpose of increasing the brightness of the fluorescent screen of a colour picture tube and for improving the colour purity so-called post focusing system has been proposed wherein an electron lens is disposed near a shadow mask for restricting the diameter of the electron beam which has passed through the shadow mask.

As diagramatically shown in FIG. 1 a conventional shadow mask comprises a rectangular frame 11 and thin metal sheet 1 supported by the frame and provided with a plurality of small perforations for passing an electrom beam. As shown in FIG. 2, the shadow mask 10 is positioned in front of a face plate having a coating of phosphor 3 applied on the inner surface of the face plate. When a potentail for post-deflection focusing is impressed across the shadow mask 10 and the phospher or fluorescent screen 3 a strong electric field is created between the shadow mask 10 and electrodes (not shown) disposed close thereto, so that the electron beam impinges upon the shadow mask through a passage a or b will be reflected by the shadow mask 10 or causes it to emit secondary electrons. An electrom beam which freely transmits through a perforation of the shadow mask as shown by a passage c will impinge upon the fluorescent screen and is then reflected by the metal back (not shown) on the phosphor film 3. These unwanted scattered electrons will be attracted to the phosphor film 3 under the influence of a strong accelerating field between the shadow mask and the fluorescent screen thus causing a reduction of contrast.

To prevent this undesirable emission of the secondary electrons it has been proposed to apply a coating of a substance which does not emit the secondary electrons, graphite for example, on the surface of the metal back on the fluorescent screen or on the front surface 4 of the shadow mask facing to the electron gun (not shown) or on the rear surface 5 facing to the fluorescent screen. However, such graphite coating can not completely eliminate the reduction of contrast on the fluorescent screen. Especially it has been impossible to sufficiently suppress the emission of the secondary electrons caused by an electron beam impinging upon the inner wall of the perforation of the shadow mask.

SUMMARY OF THE INVENTION secondary electrons emitted by the collision of an electron beam upon the shadow mask.

According to this invention, there is provided a shadow mask for use in a colour picture tube of the type including a plurality of perforations for passing an electron beam characterized in that the layers of a substance capable of suppressing the emission of secondary electrons caused by the collision of the electron beam are formed on the opposite surfaces of the shadow mask. Such a layer can also formed on the inner wall of the perforation.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings in which 7 FIG. 1 is a diagrammatic plan view, partly broken away, of a shadow mask;

FIG. 2 is an enlarged sectional view of a portion of a conventional shadow mask to explain the emission of secondary electrons caused by the impingement of electron beams;

FIG. 3 shows an enlarged sectional view of a portion of a shadow mask embodying the invention and FIG. 4 is a view similar to FIG. 4 but illustrating a modified embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 3, according to this invention, graphite is coated on the front side 4 facing to the electron gun, not shown, and the rear side 5 facing to the fluorescent screen 3 (see FIG. 2) of a metal plate 1 of a shadow mask 10 by any well-known method to form layers of graphite 6 and 7, each having a thickness of from 3 to 20 microns. Since graphite is a substance capable of preventing secondary electron emission, the layer 6 on the front side prevents the secondary electron emission caused by an electron beam a which impinges directly upon layer 6, and the layer 7 on the rear side also prevents the secondary electron emission caused by an electron beam c which impinges upon the fluorescent screen and reflected thereby or the secondary electrons emitted by the collision of such electron beam c.

In the modified embodiment shown in FIG. 4 the graphite coating is also applied onto the inner wall of the perforation of the shadow mask. The perforation 2 of the shadow mask is generally not cylindrical but generally consists of a first cup shaped section 21 having a smaller diameter on-the front side and a second cup shaped section 22 having a larger diameter on the rear side, the diameter of the second section being largest on the rear surface 5. As can be noted from FIG. 2 since the electron beam b entering the perforation 2 generally collides upon the inner surface of the larger diameter section 22, according to this invention, a graphite coating 8 is formed also on the inner surface of the large cup shaped section to a thickness of from 3 to 20 microns, for example, thereby preventing the secondary electron emission caused by the collision of an electron beam such as b.

What is claimed is:

1. In a shadow mask for use in a colour picture tube of the type including a plurality of perforations for passing an electrom beam to a colour screen, the improvement which comprises low electron emissive layers of a substance coated on said shadow mask, said substance capable of suppressing the emission of secondary electrons caused by the collision of the electron beam with said mask directly or as reflected from said also applied on the inner surface of said second cup shaped section.

3. The shadow mask according to claim 1 wherein said layers has a thickness of from 3 to 20 microns.

4. The shadow mask according to claim 1 wherein said secondary electron suppressing substance is graphite.

II i It i 

1. In a shadow mask for use in a colour picture tube of the type including a plurality of perforations for passing an electrom beam to a colour screen, the improvement which comprises low electron emissive layers of a substance coated on said shadow mask, said substance capable of suppressing the emission of secondary electrons caused by the collision of the electron beam with said mask directly or as reflected from said screen, said layers being formed on both surfaces of said shadow mask.
 2. The shadow mask according to claim 1 wherein each of said perforations comprises a first cup shaped section of a smaller diameter located on the side of said shadow mask impinged by said electron beam and a second cup shaped section of a larger diameter and located on the other side of said shadow mask and wherein said secondary electron suppression layer is also applied on the inner surface of said second cup shaped section.
 3. The shadow mask according to claim 1 wherein said layers has a thickness of from 3 to 20 microns.
 4. The shadow mask according to claim 1 wherein said secondary electron suppressing substance is graphite. 